This invention relates to a programmable controller and a communication unit therefor.
Network systems for factory automation (FA) are comprised of one or more programmable controllers (PLC) for controlling production equipments and apparatus of which the operations are controlled by the PLC, connected together to a control system network. These PLC and apparatus serve to control the production equipments by exchanging IO data among them by carrying out communications cyclically through the control network.
The PLC is formed by combining a plurality of units such as a CPU unit for carrying out calculations based on a control program, an input unit connecting to input devices such as sensors and switches for taking in their on-off signals as input signals, an output unit connecting to output devices such as actuators and relays for transmitting output signals to them, a communication unit for exchanging data with other devices connected to the network, and a power unit for supplying electric power to each unit.
There are situations in FA where a plurality of control devices such as programmable controllers are connected to the network for carrying out a cooperative control among them. For carrying out such a cooperative control, data on the input-output devices connected to the programmable controllers may be exchanged mutually among them through the network. Methods of thus exchanging data among the programmable controllers include the data exchange method of the command-response type whereby one PLC transmits a request message to another PLC for exchanging data and the data of the PLC input-output unit are received as a response, as well as the so-called datalink method whereby each PLC carries out transmission and reception of I/O data periodically, based on information that is preliminarily set.
When programmable controllers connected to a network exchange data among them, this is carried out through the communication units which are each one of the constituent units of a programmable controller. Since each communication unit is provided only with the function of transmitting and receiving data, it cannot execute any process in either of these methods requiring an access to the memory of an input-output unit such as the I/O access process of datalink and the process of getting I/O data by message transmission. Since the access to such a memory can be executed by the CPU unit which is a bus master, it was necessary for the communication unit to transmit to the CPU unit a request for I/O access.
Thus, since the I/C access process is carried out by the CPU unit whenever I/O data are exchanged, for example, the load on the CPU unit becomes heavy in the case of a high-frequency data exchange, and this may seriously affect the control which is the principal function of the CPU unit. In the case of a data exchange method which must be carried out through the CPU unit for accessing a peripheral unit such as the input-output unit, therefore, the user could not accomplish both a high-speed control of a programmable controller and a high-speed and high-frequency data exchange on the network at the same time.
Japanese Patent Publications Tokkai 5-173985 and 5-173986 have disclosed a programmable controller provided with a plurality of CPU units so as to have a multi-bus master function, these CPU units being capable of independently accessing the individual input-output units. These inventions can be utilized to provide a communication unit with the function of accessing the I/O memory of the input-output unit such that the load on the CPU units is reduced.
In view of the demand for increasing the speed of data exchange cycle and the data volume, however, there is a problem of sudden increase in the bus traffic due to the bus access by the communication devices at the time of data exchange, and both communication units and CPU units are thereby affected. This problem cannot be answered only with communication units with the capability of directly accessing peripheral units without going through the CPU unit.